1. Field of the Invention
The present invention relates to an alkaline storage battery and more particularly to an alkaline storage battery having improved charge-discharge properties.
2.Description of the Related Art
Various approaches have been proposed to improve the charge-discharge properties of alkaline storage batteries at high temperatures.
For example, it is effective to incorporate Cd in a Ni positive electrode or increase the amount of Co to be incorporated in the Ni positive electrode. However, the former approach is disadvantageous in that it is subjected to regulations on the use of Cd from the standpoint of recent environmental protection. The latter approach is disadvantageous in that it lowers the packing of active material and hence the battery capacity.
There has been proposed an approach which comprises incorporating Li and Na in the alkaline electrolyte for nickel hydride battery to improve the charge-discharge properties thereof (see Japanese Patent 2,604,282).
However, the above cited patent is disadvantageous in that since the concentration of Li in the electrolyte is 1 N or more, the depth (degree) of charging is great, resulting in the production of xcex3NiOOH that causes the expansion of the volume of the positive electrode and thus lowers the discharge voltage and capacity. On the contrary, when the content of Li is decreased, the deterioration of the discharge properties can be prevented, but the insufficiency in the content of Li makes it difficult to obtain desired charge properties.
Y (yttrium) is known as an element for improving the high temperature charge-discharge properties. Accordingly, it has been proposed that it has been proposed that Y be applied to the surface of an active material layer formed by filling a non-sintered substrate such as foamed porous material with an active positive electrode material to form a Y coat layer thereon.
However, this proposal is disadvantageous in that Y cannot be attached uniformly to the entire positive electrode including the substrate, making it impossible to allow Y to exert a sufficient effect of improving the high temperature charge properties.
The present invention has been worked out to solve the foregoing problems. It is therefore an object of the present invention to provide an alkaline storage battery which exhibits excellent charge properties as well as desired discharge properties.
The foregoing object of the present invention can be accomplished with an alkaline storage battery comprising an active positive electrode material containing nickel hydroxide as a main component and an alkaline electrolyte containing at least Li+ and Na+, characterized in that said positive electrode contains Y and/or Y compound and said alkaline electrolyte has an Li content of from not lower than 0.1 to less than 1.0 N and an Naxc2x1 content of from 0.3 to 1.5 N (N: normality).
Preferably, said positive electrode comprises Y and/or Y compound on a surface of an active positive electrode material layer comprising said active positive electrode material, which is formed on a substrate.
In some detail, the alkaline storage battery comprises as a positive electrode a porous Ni sintered substrate filled with an active material containing nickel hydroxide. Y and/or Y compound is attached to the entire positive electrode including the substrate and active material. The concentration of the alkaline electrolyte is adjusted such that the content of Li ranges from not lower than 0.1 N to less than 1.0 N and the content of Na ranges from 0.3 N to 1.5 N.
The preparation of the positive electrode is preferably accomplished by a process which comprises filling a porous Ni sintered substrate with an active material containing nickel hydroxide, dipping the substrate in a mixed salt solution containing Y, and then subjecting the substrate to alkali treatment.
Y acts to improve the high temperature charge properties.
In the present invention, the positive electrode comprises a sintered substrate. In some detail, the positive electrode is prepared by filling the substrate with an active material, and then dipping the substrate in a mixed salt containing yttrium (e.g., yttrium nitrate-nickel solution). In this dipping process, Y is attached almost uniformly to the entire positive electrode including the substrate and active material in the form of yttrium hydroxide so that a Y coat layer is formed over the entire positive electrode, making it possible to exert excellent high temperature charge properties even if the content of Y is small.
Accordingly, even when the concentration of Li in the alkaline electrolyte falls below 1 N, the presence of the Y coat layer formed attached to the entire positive electrode makes it possible to provide excellent high temperature charge properties.
Further said positive electrode is formed by a process which comprises a dipping step of dipping a sintered nickel electrode in a salt containing yttrium and nickel so that it is filled with yttrium and an activating step of dipping said electrode in an aqueous solution of alkali.